There have been known nitride system semiconductor light emitting devices containing GaN and the like. For example, white LEDs, each of which is a combination of a blue LED (light emitting diode) containing GaN and an yellow light emitter, are widely used as backlights of liquid crystal displays (LCDs) of mobile phones and the like. Moreover, the white LEDs are characterized by low power consumption and long lifetime. Accordingly, the white LEDs will be further expected as light sources alternative to fluorescent and incandescent lamps in the future and are being increasingly studied and developed.
However, an LED outputs only about several watts and is therefore desired to increase in brightness. Conceivable means for increasing the brightness is increasing the internal quantum efficiency of the LED, increasing the extraction efficiency of extracting light emitted from an active layer to the outside, or the like. Since the light refractive index of GaN (about 2.5) is extremely higher than that of air (about 1.0) in particular, the total reflection angle at the interface between the LED and air is small, and the light extraction efficiency is very small (for example, about 10%).
Accordingly, various techniques to increase the efficiency of extracting light emitted by a semiconductor light emitting device are disclosed.
For example, Patent Literature 1 discloses a semiconductor light emitting device including a semiconductor stack with the surface of the top layer roughened.
This semiconductor light emitting device includes a semiconductor stack having a p-GaN layer provided, an active layer, and an n-GaN layer on a substrate. The upper surface of the n-GaN layer formed in the top layer of the semiconductor stack has conical protrusions formed to be roughened. Roughening the upper surface of the n-GaN layer as described above can prevent light emitted by the active layer from being reflected at the interface between the n-GaN layer and air by Snell's law, thus increasing the amount of light emitted to the outside.
In the method of manufacturing this semiconductor light emitting device, first, using MOCVD (metal organic chemical vapor deposition), the n-GaN layer, active layer, p-GaN layer are sequentially stacked on a growth substrate (a sapphire substrate) to form the semiconductor stack. Subsequently, a p-side electrode is formed on the p-GaN layer, and the semiconductor stack is then re-attached to a supporting substrate so that the p-side electrode is positioned on the supporting substrate (silicon substrate) side.
Next, on the upper surface of the n-GaN layer at the top of the semiconductor stack reattached to the supporting substrate, an n-side electrode is formed. Thereafter, the upper surface of the n-GaN layer is formed into a rough surface with conical protrusions by PEC (photo-enhanced chemical) etching using KOH solution and a xenon/mercury lamp.